Medical Tests MCAT-TEST Online Practice Questions and Exam Preparation

Medical Tests MCAT-TEST Online Questions & Answers

• Question 811:

  Just as the ingestion of nutrients is mandatory for human life, so is the excretion of metabolic waste products. One of these nutrients, protein, is used for building muscle, nucleic acids, and countless compounds integral to homeostasis. However, the catabolism of the amino acids generated from protein digestion produces ammonia, which, if not further degraded, can become toxic. Similarly, if the same salts that provide energy and chemical balance to cells are in excess, fluid retention will occur, damaging the circulatory, cardiac, and pulmonary systems.

  One of the most important homeostatic organs is the kidney, which closely regulates the excretion and reabsorption of many essential ions and molecules. One mechanism of renal function involves the secretion of antidiuretic hormone (ADH).

  Diabetes insipidus (DI), is the condition that occurs when ADH is ineffective. As a result, the kidneys are unable to concentrate urine, leading to excessive water loss. There are two types of DI -- central and nephrogenic. Central DI occurs when there is a deficiency in the quantity or quality of ADH produced. Nephrogenic DI occurs when the kidney tubules are unresponsive to ADH. To differentiate between these two conditions, a patient's urine osmolarity is measured both prior to therapy and after a 24-hour restriction on fluid intake. Exogenous ADH is then administered and urine osmolarity is measured again. The table below gives the results of testing on four patients. Assume that a urine osmolarity of 285 mOsm/L of H2O is normal.

  

  Which of the following substances would NOT be found in appreciable quantity in the urine of a healthy individual?

  A. Albumin
  B. Sodium
  C. Urea
  D. Potassium
  A. Albumin

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  Explanation/Reference:

  Albumin is a protein produced by the liver. It is very important in maintaining the plasma osmolarity of the blood. The glomerulus, which is the network of blood vessels enveloped by the part of the nephron known as Bowman's capsule, has small holes in its endothelial lining called fenestrations. When proteins travel through the glomerulus, they are prevented from entering the nephron due to the size of the fenestrations and the negative electrical charge of glycosylated proteins lining the fenestrations. Therefore, albumin is not normally found in urine because it is prevented from entering the nephron, and so choice A is the right answer. The presence of protein in a urine sample is typically a sign of renal disease. Each of the other choices are normally found in appreciable quantity in the urine. Urea, choice C, is the primary excretory byproduct of the urea cycle. Sodium and potassium homeostasis are kept in balance by the regulation of their excretion in urine.

• Question 812:

  Studies of photosynthesis began in the late eighteenth century. One scientist found that green plants produce a substance (later shown to be oxygen) that supports the flame of a candle in a closed container. Several years later it was discovered that a plant must be exposed to light in order to replenish this flame- sustaining "substance". Soon another discovery showed that the oxygen is formed at the expense of another gas, carbon dioxide.

  In 1804, de Saussure conducted experiments revealing that equal volumes of carbon dioxide and oxygen are exchanged between a plant and the air surrounding it. De Saussure determined that the weight gained by a plant grown in a pot equals the sum of the weights of carbon derived from absorbed carbon dioxide and water absorbed through plant roots. Using this information, de Saussure was able to postulate that in photosynthesis carbon dioxide and water combine using energy in the form of light to produce carbohydrates, water, and free oxygen. Much later, in 1845, scientists' increased understanding of concepts of chemical energy led them to perceive that, through photosynthesis, light energy is transformed and stored as chemical energy.

  In the twentieth century, studies comparing photosynthesis in green plants and in certain sulfur bacteria yielded important information about the photosynthetic process. Because water is both a reactant and a product in the central reaction, it had long been assumed that the oxygen released by photosynthesis comes from splitting the carbon dioxide molecule. In the 1930s, however, this popular view was decisively altered by the studies of C. B. Van Niel. Van Niel studied sulfur bacteria, which use hydrogen sulfide for photosynthesis in the same way that green plants use water, and produce sulfur instead of oxygen. Van Niel saw that the use of carbon dioxide to form carbohydrates was similar in the two types of organisms. He reasoned that the oxygen produced by green plants must derive from water -- rather than carbon dioxide, as previously assumed -- in the same way that the sulfur produced by the bacteria derives from hydrogen sulfide. Van Niel's finding was important because the earlier belief had been that oxygen was split off from carbon dioxide, and that carbon then combined with water to form carbohydrates. The new postulate was that, with green plants, hydrogen is removed from water and then combines with carbon dioxide to form the carbohydrates needed by the organism.

  Later, Van Niel's assertions were strongly backed by scientists who used water marked with a radioactive isotope of oxygen in order to follow photosynthetic reactions. When the photosynthetically-produced free oxygen was analyzed, the

  isotope was found to be present.

  Which of the following statements about photosynthesis would most probably NOT have been made by de Saussure?

  A. It involves an exchange of equal quantities of gases.
  B. It results in the conversion of light energy to chemical energy.
  C. It produces oxygen.
  D. It requires light.
  B. It results in the conversion of light energy to chemical energy.

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  Explanation/Reference:

  This is an inference question, which asks for the identification of a statement that probably would not have been made by de Saussure in relation to photosynthesis. De Saussure is discussed in the second paragraph. According to the last sentence of the paragraph, the conversion of light energy into chemical energy was not well understood until after de Saussure's work. According to the previous sentence, de Saussure was aware only that light supplied energy to plants. So it's unlikely that the statement made in choice (B) would have been made by de Saussure. Each of the other choices is mentioned earlier in this paragraph or in paragraph 1 as having been known to or discovered by de Saussure.